Method for Fabricating a Polymer L.E.D.

ABSTRACT

A light-emissive device is prepared by depositing a polymer layer on a substrate. The deposition process utilizes a formulation comprising a conjugated polymer dissolved in a solvent, the solvent including a trialkyl-substituted aromatic hydrocarbon wherein at least two of the alkyl substituents are ortho to one another. The deposition of the polymer layer on the substrate may be accomplished by an ink-jet method.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/339,361filed Jan. 25, 2006, which was a continuation of application Ser. No.10/362,431 filed Jul. 29, 2003, now U.S. Pat. No. 7,023,012, claimingthe priority of PCT/GB01/00852 filed Feb. 28, 2001, and entitled:“Formulation for depositing a conjugated polymer layer.”

The present invention relates to a formulation for depositing aconjugated polymer layer in a light-emissive device (LED).

Light-emissive devices using as the light-emissive layer asemiconductive conjugated polymer are known. FIG. 1 shows theconstruction of a simple light-emissive device. A glass or plasticssubstrate 2 is coated with an anode layer 4, for example in the form ofindium tin oxide. The anode can be patterned in the form of elongatestrips. The anode layer may be coated with a hole transport layer. Alight-emissive polymer layer 6 is then deposited followed by thedeposition of an electron transport layer. The device structure is thencompleted by the deposition of a cathode layer 8. By way of example, thecathode layer can be calcium or aluminium. The cathode layer 8 can bepatterned in crosswise strips to define pixels P where the anode andcathode overlap. Alternatively with an unpatterned cathode, lightemissive strips can be defined. Further alternatively, the pixels may bedefined on an active matrix back-plane and the cathode may be patternedor unpatterned. When an electric field is applied between the anode andcathode, holes and electrons are injected into the light-emissivepolymer layer 6. The holes and electrons recombine in the polymer layerand a proportion decay radiatively to generate light.

The hole transport layer can be comprised generally of any compoundcapable of sustaining hole transport. Examples of suitable materials areorganic conductors such as the following conducting polymers:polyaniline, polyethylenedioxythiophene and other polythiophenes,polypyrrole, etc. in their doped forms. Other alternative materials areconjugated polyamines and also low molecular weight amines such as TPD.

The light-emissive layer may comprise any molecular or polymericcompounds which are capable of sustaining charge carrier transport andalso capable of light emission under device driving conditions. Examplesinclude fluorescent organic compounds and conjugated polymers such asAlq3, polyphenylenes and derivatives, polyfluorenes and derivatives,polythiophenes and derivatives, polyphenylene vinylenes and derivatives,polymers containing heteroaromatic rings, etc.

The electron transport layer may generally comprise any material capableof sustaining electron transport. Examples include perylene systems,Alq3, polyfluorenes or polyfluorene copolymers, polymers containingheteroaromatic rings, etc.

The device may contain any combination of the above layers provided itincludes at least one light-emissive layer.

OLEDs are described in U.S. Pat. No. 5,247,190 or in U.S. Pat. No.4,539,507, the contents of which are incorporated herein by reference.In U.S. Pat. No. 5,247,190 the active organic layer is a light-emissivesemiconductive conjugated polymer and in U.S. Pat. No. 4,539,507 theactive organic layer is a light-emissive sublimed molecular film.

Conventionally, the polymer layer was typically deposited byspin-coating or metered blade-coating a polymer solution onto the anodeand then either allowing the solvent to evaporate at RTP, or driving offthe solvent using heat treatment and/or reduced pressure. The polymerwas the light-emissive polymer itself cast directly from solution, or aprecursor to the polymer, which is converted to the light-emissivepolymer during a heat treatment step. The polymer layer can comprise ablend of two or more materials, such as a blend of two or more polymers.

The polymer layer(s) may also be deposited by supplying asolution-processible material including the polymer through a pluralityof elongate bores, either through the effect of gravity or underpressure or utilising the effect of surface tension. This facilitatesdirect deposition or patterning of the polymer films as required.

Deposition of the polymer layer(s) by means of an ink-jet method is aparticularly preferred technique. The ink-jet method is described, forexample, in EP0880303A1, the content of which is incorporated herein byreference.

It is desirable to use material formulations with which thin polymerfilms exhibiting excellent emission uniformity can be produced.

The present applicant's own previous application PCT/GB00/03349discloses examples of formulations for depositing conjugated polymersbased on polyalkylated benzenes such as cymene and isodurene.

It is also an aim of the present invention to provide formulations thatfacilitate the direct deposition of patterned polymer films. Inparticular, it is an aim of the present invention to provide aformulation with which conjugated polymer films that exhibit improvedemission uniformity can be deposited.

According to a first aspect of the present invention, there is provideda formulation for depositing a polymer layer in the production of alight-emissive device, the formulation including a conjugated polymerdissolved in a solvent, the solvent including a dialkyl- ortrialkyl-substituted aromatic hydrocarbon, at least two of the alkylsubstituents being ortho to one another.

The solvent preferably includes one or more of 1,2-dimethyl benzene,1,2,3-trialkylbenzene and 1,2,4-trialkylbenzene. 1,2-dimethyl benzeneand 1,2,3-trialkylbenzene are particularly preferred.

According to a second aspect of the present invention, there is provideda formulation for depositing a polymer layer in the production of alight-emissive device, the formulation including a conjugated polymerdissolved in a solvent, the solvent including a trialkylbenzene.

According to a third aspect of the present invention there is provided amethod of depositing a polymer layer by supplying a solution processibleformulation according to the first or second aspects of the presentinvention via a plurality of elongate bores onto a substrate.

According to a fourth aspect of the present invention, there is provideda method of forming a polymer layer on a substrate comprising the stepof depositing a formulation according to the first or second aspects ofthe present invention on a substrate by an ink-jet method.

For the purposes of this application, the term “polymer” includeshomopolymers and copolymers, and also includes layers of polymer blendsas well as layers of single polymers; and the term “polymer layer”includes a layer of relatively small dimensions, such as a layer havingdimensions corresponding to a single pixel in a pixelated display, aswell as layers of larger dimensions.

For the purposes of this application, the term “alkyl” refers to linearor branched alkyl substituents having up to 6 carbon atoms.

The resulting formulation preferably has a surface tension in the rangeof 25 to 40 mN/m at 20° C., a viscosity of 2 to 8 cPs at 20° C., and aw/v polymer concentration of 0.25 to 1.5%, preferably 0.5 to 1%. Aformulation having such properties is particularly suited for depositionby ink-jet printing. The formulation should also have a contact angleappropriate for the substrate on which the deposition is carried out.For example, in the case of a polyimide substrate, the contact angleshould be in the range of 30 to 60 degrees.

The formulation may include one or more additional types of solventsprovided that the resulting formulation has the necessary fluidproperties for the particular deposition technique. The presence of evena relatively small proportion of, for example, one or more trialkylbenzenes in the formulation can serve to improve the stability of theformulation and/or the fluid properties of the formulation.

For the purposes of this application, the term conjugated polymer refersto polymers, including oligomers such as dimers, trimers etc., which arefully conjugated (i.e. are conjugated along the entire length of thepolymer chain) or are partially conjugated (i.e. which includenon-conjugated segments in addition to conjugated segments).

The polymer may have an average molecular weight in the range of 10,000to 500,000, more particularly in the range of 10,000 to 300,000.

The polymer may be a polymer suitable for use in a light-emissive layer,a hole transport layer or an electron transport layer in an organiclight-emissive device.

In a preferred example, the conjugated polymer can be a light-emissivepolymer, hole transport polymer or electron transport polymer itself, ora precursor to a light-emissive polymer, hole transport polymer orelectron transport polymer. The conjugated polymer or its precursor canbe any suitable polymer, and in particular can be any one of thefollowing:

a) conducting polymers such as polyaniline (PANI) and derivatives,polythiophenes and derivatives, polypyrrole and derivatives,polyethylene dioxythiophene; doped forms of all these and particularlypolystyrene sulphonic acid-doped polyethylene dioxythiophene (PEDT/PSS);

b) solution processible charge transporting and/orluminescent/electro-luminescent polymers, preferably conjugated polymerssuch as: polyphenylenes and derivatives, polyphenylene vinylenes andderivatives, polyfluorenes and derivatives, triaryl containing polymersand derivatives, precursor polymers in various forms, copolymers(including the above-named polymer classes), generally random and blockcopolymers, polymers with the active (charge transporting and/orluminescent) species attached as side-groups to the main chain,thiophenes and derivatives, etc.

It is also envisaged that the present invention is also applicable toformulations comprising other compounds such as solution processiblemolecular compounds including spiro-compounds, such as described forexample in EP-A0676461; and other inorganic compounds, e.g.solution-processible organometallic precursor compounds to fabricateinsulators or conductors.

The conjugated polymer may, for example, comprise a fluorene-basedpolymer, or a copolymer containing fluorene units with other units suchas triarylamine units or benzylthiadiazole (BT) units, or a blend of twoor more of such polymers.

For a better understanding of the invention and to show how the same maybe carried into effect, embodiments of the present invention aredescribed hereunder, by way of example only, with reference to theaccompanying drawings in which:

FIG. 1 is a diagram of a light-emissive device;

FIG. 2 is a diagram illustrating a method of depositing various polymerlayers; and

FIG. 3 shows the structure of recurring units and polymers used in theexamples.

FIG. 2 illustrates a deposition technique for depositing a polymer layer6 onto the patterned anode 4. A plurality of elongate bores 10 areillustrated, each aligned with a respective trough 8. The elongate bores10 are connected via a conduit 12 to a reservoir 14 holding the solutionto be deposited. The solution is supplied through the elongate bores 10under pressure or by gravity to deposit the light emitting polymer layer6.

Specific formulations according to the present invention are discussedherebelow.

EXAMPLE 1

The first exemplified formulation is 1% w/v of a triblend containingapproximately 14 wt. % of a ternary polymer containing fluorene (F8)units (shown in FIG. 3 a), benzylthiadiazole (BT) units (shown in FIG. 3b) and triarylene units shown in FIG. 3 c, 56 wt. % of F8BT (shown inFIG. 3 d) and 30 wt. % TFB (shown in FIG. 3 e) in 1,2,4trimethylbenzene. The resulting formulation had a viscosity of 3.8 cpsat 20° C. and a surface tension of 28 mN/m at 20° C.

EXAMPLE 2

According to the second example, the formulation comprises 1% w/v of thetriblend referred to in Example 1 in a solvent mixture containing 20vol. % 1,2,4-trimethylbenzene and 80 vol. % xylene. The formulation hada solution viscosity of 2.9 cps at 20° C. and a surface tension of 28mN/m at 20° C.

A sample of the formulation was deposited on a substrate and was vacuumdried. The PL efficiency of the thus produced polymer layer was found tobe 43%. Drying a layer of the same formulation at 80° C. for 13.5 hoursresulted in a polymer layer having a PL efficiency of 39%.

EXAMPLE 3

According to a third example, the formulation comprises 1% w/v of thetriblend referred to in Example 1 in a solvent mixture containing 50vol. % 1,2,4-trimethylbenzene and 50 vol. % xylene. The formulation hada solution viscosity of 2.82 cps at 20° C. and a surface tension of 28mN/m at 20° C.

The addition of the trimethyl benzene is found to significantly increasethe stability of the formulation.

1,2-dimethyl benzene, 1,2,3-trimethylbenzene and 1,2,4-trimethylbenzenewere also found to be good solvents for a fluorene-thiophene polymerhaving an average molecular weight of about 266,000.

1. A method of fabricating a polymer light-emitting diode comprising:providing a substrate for a polymer light-emitting diode; dissolving aconjugated polymer in a solvent comprising a dialkyl- ortrialkyl-substituted aromatic hydrocarbon wherein at least two of thealkyl substituents are ortho to one another; and, depositing thedissolved, conjugated polymer on said substrate.
 2. A method as recitedin claim 1 wherein the depositing is accomplished by ink-jet printing.3. A method as recited in claim 1 wherein the solvent comprises1,2-dimethylbenzene.
 4. A method as recited in claim 1 wherein thesolvent comprises 1,2,3-trimethylbenzene.